Introduce explicit base render backend type

The main idea behind the render backend is to decouple low level bits from scenes. The end goal is to make the render backend provide render targets where the scene can render.

Design-wise, such a split is more flexible than the current state, for example we could start experimenting with using qtquick (assuming that the legacy scene is properly encapsulated) or creating multiple scenes, for example for each output layer, etc.

So far, the RenderBackend class only contains one getter, more stuff will be moved from the Scene as it makes sense.